1. Field of the Invention
The present invention relates to a surface acoustic wave (SAW) device.
2. Description of the Background Art
For example, the SAW devices include an SAW resonator, a transversal type SAW filter and a resonator type SAW filter which are selectively used according to application. In the following, an SAW filter, for example, as one of the SAW devices will be described.
FIG. 28 is a perspective view schematically showing the structure of an SAW filter. Referring to FIG. 28, the basic structure of the SAW filter 15 is a four-terminal structure which has a piezoelectric substrate 12 and a pair of comb-shaped electrodes 13 for surface wave excitation and reception formed on a surface of piezoelectric substrate 12. This type of electrode 13 is called an interdigital electrode, and this type of a transduction element is called an IDT (interdigital transducer).
In general, when an impulse voltage is applied to comb-shaped electrode 13 for oscillation, the piezoelectric effect causes opposite phase strain between adjacent electrodes 13 as shown in FIG. 29, and an SAW is excited. The SAW propagates on the surface of piezoelectric substrate 12. A surface electric charge is caused on piezoelectric substrate 12 by the strain, which is brought about by the surface wave, and extracted as an electric signal by comb-shaped electrode 13 for reception.
Conventionally, an SAW device such as SAW filter 15 has a structure in which an electrode corresponding to each device is arranged on the surface of piezoelectric substrate 12 as shown in FIG. 30. The characteristics of SAW device 15 largely depend on those of piezoelectric substrate 12, and piezoelectric substrate 12 is also selectively used according to application. Table 1 indicates typical materials for piezoelectric substrate 12 and the characteristics of an SAW which piezoelectric substrate 12.
As can be seen from Table 1, the crystal substrate has low and superior temperature characteristics but it has small electro-mechanical coupling coefficients (K2). Conventionally, a 128xc2x0 Y-X LN substrate (expressed by Eulerian angles of (0xc2x0, 38xc2x0, 0xc2x0) LN) is generally used for an LiNbO3 (LN) substrate. However, the conventional LN substrate has large values of K2 but it has inferior temperature characteristics such as a temperature coefficient of delaytime (TCD). On the other hand, the Li2B4O7 substrate has intermediate characteristics between those of the crystal substrate and the LN substrate.
As described above, each substrate has both advantageous and disadvantageous points and is selectively used according to application of the device. In recent years, video devices such as a TV and telecommunication devices such as a mobile phone have been developed, and therefore SAW devices used therefor need to have characteristics superior than ever.
Here, the Eulerian angles in Table 1 will be described with reference to FIG. 31.
Referring to FIG. 31, the Z axis is first used as a rotational axis to rotate the X axis in the Y axis direction by xcfx86 so as to provide a first axis. Then, the first axis is used as a rotational axis to rotate the Z axis counterclockwise by xcex8 so as to provide a second axis. A substrate is obtained by cutting a substrate material at a plane orientation such that the second axis is normal to the plane and the first axis is on the plane. In the substrate which is cut at the orientation, the second axis is used as a rotational axis to rotate the first axis counterclockwise by "psgr" to provide a third axis. The third axis is employed as an SAW propagation direction. It is noted that an axis perpendicular to the third axis on the plane is a fourth axis. In this manner, the Eulerian angles (xcfx86, xcex8, "psgr") are defined.
Recently, attaining smaller SAW devices have been requested. It is considered in general that a device with a large bandwidth can be designed more easily as the value of K2 of a piezoelectric substrate is higher. Furthermore, a substrate with a large value of K2 makes it possible to reduce the number of electrodes, and it is advantageous for providing a smaller SAW device. Conventionally, however, K2 is a value inherent in a substrate material although it more or less increases or decreases with Eulerian angles employed. Conventionally, it is therefore necessary to begin with the development of a substrate material in order to obtain an SAW device substrate having a large value of K2. If this can be achieved by using conventional materials, that can greatly contribute to the technical field.
Here, the center frequency fO of an SAW device is determined by fO=V/xcex (V: the propagation velocity of a surface acoustic wave, xcex: the electrode pitch of an IDT). Therefore, if the Li2B4O7 substrate having a higher propagation velocity V than that of a crystal substrate or the like is simply used for manufacturing a device having the same center frequency fO, the electrode pitch xcex of the IDT needs to be increased and, as a result, the SAW device itself has to be made larger. In short, a substrate with a lower propagation velocity V is advantageous for attaining a smaller SAW device itself.
Therefore, the present invention aims to attaining a smaller SAW device as a common object. For the purpose, the present invention has a first object of obtaining, by using a conventional material, an SAW device substrate which has larger K2 advantageous for attaining a smaller and higher performance SAW device, and a second object of obtaining an SAW device substrate which has intermediate K2 between those of a crystal substrate and an LN substrate and has a low propagation velocity advantageous for attaining a smaller SAW device.
The inventors conducted an extensive study based on the above objects and found out that larger K2 can be obtained by setting Eulerian angles and the value of KH in prescribed ranges, wherein K is a value found by dividing 2xcfx80 by an electrode pitch, H is a piezoelectric substrate thickness, and KH is a product of K and H. In the present invention, the above object is attained by forming LN, which is expressed as (18-30xc2x0, 80-100xc2x0, 35-75xc2x0) in terms of Eulerian angles, on a glass surface so as to provide a (18-30xc2x0, 80-100xc2x0, 35-75xc2x0) LN/glass structure substrate.
In the (18-30xc2x0, 80-100xc2x0, 35-75xc2x0) LN/glass structure substrate, it is more preferable that much higher K2 can be obtained by setting a KH parameter so that 2.8xe2x89xa6KHxe2x89xa63.8.
In the present invention, the above described objects are also achieved by forming an Li2B4O7 layer on a glass surface to obtain an Li2B4O7/glass structure substrate. More preferably, such Li2B4O7 that is expressed as (0-45xc2x0, 85-95xc2x0, 85-95xc2x0) in terms of Eulerian angles is used in the Li2B4O7/glass structure substrate. More preferably, the KH parameter is set at about 0.5 to obtain an SAW device substrate having a lower propagation velocity.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.